Caltech Scientists Offer Theory of Ganymede's Oxygen and Ozone

Tucson, Arizona — When Galileo discovered Ganymede four centuries ago, little did he suspect that the third satellite from Jupiter might be glazed over with the very substance he was breathing.

It took modern astronomical instruments and chemical knowledge for scientists to detect the oxygen and ozone that coat Ganymede. Now, two planetary scientists affiliated with the California Institute of Technology have developed a theory to account for the presence of the substances, as well as the mechanism by which their concentrations are maintained.

Dr. Yuk Yung and Dr. Ming-Taun Leu, in a presentation at the annual Division for Planetary Science (DPS) meeting of the American Astronomical Society, today provide an explanation for the amount of oxygen and ozone detected by ground-based telescopes and the Hubble Space Telescope. According to Yung, the oxygen and ozone are remnants of the primordial water that became part of Ganymede when the solar system was formed 4.5 billion years ago.

Yung, professor of planetary science at Caltech, theorizes that the water ice on Ganymede has since time immemorial been attacked by two sources: ultraviolet light from the sun, and ions thrown off by the volcanic activity of the sister Jovian satellite Io. Both sources of disturbance have the effect of blasting the water molecules apart. Once the hydrogen and oxygen of the water are separated, the lighter, energetic hydrogen ions blast away from the light gravity of Ganymede into outer space, while the heavier oxygen molecules settle back onto the surface.

Yung also offers a theory to account for the relatively high concentration of molecular oxygen and ozone on Ganymede's surface. While each is present at about one-thousandth its concentration on Earth, both are far and away more prevalent than on any other body in the solar system except Mars, which by coincidence has roughly the same concentrations.

"So far, Earth, Mars, and Ganymede are the only bodies in the solar system with ozone," said Yung in an interview prior to the DPS conference.

Yung's theory of oxygen and ozone concentrations on Ganymede assumes the presence of a vast structure of tiny surface cracks in the ice where the frozen oxygen and ozone can reside. To test the hypothesis that such cracks indeed exist, Yung's colleague, Ming-Taun Leu of Caltech's Jet Propulsion Laboratory, devised an experiment in which the conditions of Ganymede could be simulated.

The results showed that such cracks could indeed occur in the ice matrix, and that even a very thin surface coating of ice could harbor the high concentrations of oxygen and ozone seen on Ganymede.

As for the destruction of the water molecules, Yung explains that the manner in which the molecules are blown apart can account for the ratio of oxygen to ozone, as well as the net amount of oxygen atoms on the surface. Light energy from the sun, for example, tends to split a molecule of oxygen (O2) into two oxygen atoms, and a molecule of ozone (O3) into a molecule of oxygen and a single oxygen atom.

An oxygen ion that finds its way from Io's volcanoes to Ganymede, however, tends to turn a molecule of oxygen it hits into a molecule of ozone. Or, if it hits an existing molecule of ozone, the particle from Io tends to combine with the three atoms to make two molecules of O2. By this scenario, the energy expended and consumed in these reactions accounts for the amount of oxygen as compared to the amount of ozone.

Yung says that his work is very basic in nature and has little to do with the present amount of oxygen on Earth, because our own planet is dependent on living processes for most of its oxygen. Nonetheless, he says that the study of Ganymede can perhaps lead to answers about how oxygen might have arisen on Earth and Mars before life began.